The emissions from vehicles and internal combustion engines generally are the subject of ever-tightening regulation around the world. Concern about global warming associated with CO2 emissions has led to fiscal incentives in a number of countries to reduce CO2 emissions from vehicles. Increasingly, therefore, private cars and light commercial vehicles are being powered by light duty diesel engines, which have relatively low fuel consumption and relatively low CO2 emissions.
Amongst the strategies being adopted to improve both fuel consumption and emissions for both gasoline spark ignition engines and compression ignition (e.g. Diesel) engines is “stop-start”. With a stop-start system, when the vehicle halts for more than a few seconds, the engine is stopped entirely. When the driver needs to move off once more, e.g. depressing the clutch, moving the gear stick, turning the powered steering wheel or, in automatic or semi-automatic vehicles, shifting to “drive”, causes the engine to be re-started. Although this causes more load on the battery and starter motor, so that these need to be upgraded, there can be significant savings. The savings in tests under the New European Drive Cycle may, depending on the stop-start system adopted, be of the order of up to 5% of fuel consumption and up to 8% of CO2 emissions. Urban authorities are keen on reducing emissions in towns and cities, and from heavy traffic, so it is likely that stop-start systems will be included in many new vehicles.
Light duty Diesel engines are becoming even more efficient, with electronic control modules and injection technology being combined with mechanical improvements. This means that the exhaust gas temperatures are very much lower than with gasoline engines or heavy duty (truck and bus) diesel engines. Under light load, for example in urban use, and when “coasting” in gear, little or no fuel is being used by such latest design light duty Diesel engines, and the exhaust gas temperatures may, be no greater than about 100-200° C. Despite these low temperatures, advanced catalyst technology can achieve light-off during the New European Drive Cycle, during real-life city driving conditions, low-speed accelerations and steady driving conditions. “Light-off” may be defined as the temperature at which a catalyst catalyses a reaction at a desired conversion activity. For example “CO T50” is a temperature at which a particular catalyst causes the conversion of carbon monoxide in a feed gas, for example to CO2, with at least 50% efficiency. Similarly, “HC T80” is the temperature at which hydrocarbon, perhaps a particular hydrocarbon such as octane or propene, is converted, e.g. to water vapour and to CO2 at 80% efficiency or greater.
However, under certain circumstances low exhaust gas temperatures can mean that the Diesel Oxidation Catalyst (DOC) may be unable to operate effectively. That is, the DOC may be unable to achieve or to maintain “light-off”.
For vehicles not fitted with an engine “stop-start” system, an additional problem arising from operation of the engine under such light load conditions is that whilst the engine is operating, relatively cool exhaust gases, comprising mostly air, continue to pass from the engine through the DOC or other catalyst. This flow of cool gases can cool the DOC to below light-off temperatures. When load is reapplied, for example upon acceleration, the catalyst is unable to meet the desired conversion of the pollutant gases immediately, with the result that emissions of pollutants may be above the regulated levels for a period. In due course, the higher temperature exhaust gases raise the catalyst temperature above light-off temperature once more.
One known DOC design is disclosed in our WO 2007/077462 and comprises a flow-through monolith comprising (numbering from upstream to downstream) first, second and third platinum-group metal-containing washcoat zones. The platinum group metal loading in each of the first and third zones is greater than in the second zone, which is spatially disposed between the first and third zones. The third zone, that is the zone which, when in use, is disposed furthest from the engine, may include a washcoat having a higher thermal mass than the first and second zones, for example by using a thicker washcoat or a washcoat material having an inherently higher thermal mass, such as densified zirconia. Densified zirconia can have a density of 3.5 g/cm3. The three-zone arrangement is designed to maintain catalyst performance at an overall reduced total platinum group metal cost.